We don't know very much about older Chinese martial arts, and especially little of a technical nature, for a number of reasons:

1. Chinese martial arts attracted little attention from contemporary Chinese scholars, so little was recorded about them.

2. Technical details of martial arts are difficult to convey in text, or text supplemented by pictures. Teaching is generally direct instruction, rather than by book, and it is necessary to practice the techniques in order to properly learn them (this is why it is martial "arts", "art = skill acquired through practice").

3. Some material that was written about martial arts is fiction dressed as fact: "history" simply repeating myths, or "history" that consists of fabricated origin stories.

4. Chinese martial arts have attracted little attention from modern academia.

For discussion of these problems, see Henning, S. E. "Academia Encounters the Chinese Martial Arts." China Review International, vol. 6 no. 2, 1999, pp. 319-332. doi:10.1353/cri.1999.0020

See also Henning, Stanley E. “The Chinese Martial Arts in Historical Perspective.” Military Affairs, vol. 45, no. 4, 1981, pp. 173–179. JSTOR, www.jstor.org/stable/1987462.

For a broad survey of the history of Chinese martial arts, see Peter Lorge, Chinese Martial Arts: From Antiquity to the Twenty-First Century:

https://books.google.com/books?hl=en&lr=&id=Nzjv0uPqUyMC

The best-known of old Chinese martial arts is archery. First, archery was a fit pursuit for gentlemen and scholars. Second, it was the militarily most important martial art until the Ming Dynasty. For an excellent collection of Chinese sources on archery, in English translation, see Stephen Selby, Chinese Archery:

https://books.google.com/books/about/Chinese_Archery.html?id=wY3sAQAAQBAJ

A complete late Ming archery manual is available in English translation, Jie Tian and Justin Ma, The Way of Archery: A 1637 Chinese Military Training Manual:

https://books.google.com/books/about/The_Way_of_Archery.html?id=6ca5oAEACAAJ

There is also material about archery in Science and Civilisation in China, Vol. 5, Part 6.

The unarmed martial arts are what we know the least about. Before the late Ming printed manuals, some of which feature unarmed fighting (they mainly cover armed martial arts), we know very little (there are some mentions of wrestling). From the late Ming onwards, we have more sources. A good coverage of the late Ming and Qing evolution of unarmed martial arts is given by Meir Shahar, The Shaolin Monastery: History, Religion, and the Chinese Martial Arts:

https://books.google.com/books?hl=en&lr=&id=KiNEB0H6S0EC

For a selection of late Ming and later manuals, see https://www.chineselongsword.com/

Also, Brian Kennedy and Elizabeth Guo, Chinese Martial Arts Training Manuals: A Historical Survey: https://books.google.com/books/about/Chinese_Martial_Arts_Training_Manuals.html?id=L5FVYobC7VsC

Was the Jian for instance reeally used the way modern practitioners use it?

This introduces another difficulty. Much of what we know about old Chinese martial arts is some of their training methods/tools. Specifically, what we know might be limited to the forms (kata, patterns) used in training. Forms are not necessarily what the martial art would look like in fighting. For example, compare fighting in modern sanda/sanshou (Chinese kickboxing) with modern Chinese martial arts forms. Different martial arts styles can be thought of as different ways of learning how to fight, rather than purely as different way of fighting.

As well as this "kickboxer effect" (all fighters fight like kickboxers when under kickboxing rules), there are also many theatrical/performance elements in many forms. This is not just a modern phenomenon; Qing Dynasty martial artists would earn money as street performers.

There are techniques in common between modern forms and the forms in the late Ming manuals. For modern Chinese swordsmanship that might look like old Chinese swordsmanship, the techniques in these forms, used with (simulated) weapons of suitable weight, practiced against resisting opponents, have seen use in full-contact tournaments. You can find video if you search for "Traditional Chinese Sword League" and "Jian Fa Sword Tournament". While this fighting is very much in a sporting context, it is a better model for historical jian fighting than modern wushu forms performance.